There are numerous examples of physical security fences known in the prior art. In addition, the prior art is replete with examples of physical security fences that incorporate optic fibers thereinto, at least in part, to detect breaches of the physical fence by sounding an alarm when optical fibers are severed. Several examples are discussed below.
U.S. Pat. No. 4,365,239 describes an intrusion warning system for protecting a wall or a fence, in particular a chain link fence, against intrusion (via cutting) or by climbing over same. The system combines a shielded cable 2 with a chain link fence 1. The fence 1 is intended to extend around the perimeter of an area to be protected. The length of the shielded cable 2 may be as long as 1,000 feet (304.8 m) in the described example. The shielded cable 2 is an electrical coaxial cable with an inner conductor surrounded by an outer, shielded conductor with a layer of insulating dielectric material therebetween. In operation, a signal is sent through the coaxial cable. An attempt to break through the fence disrupts the electrical signal, triggering an alarm.
U.S. Pat. No. 4,399,430 describes an intruder detection security system including a security fence 2 made from a plurality of elongated members 4, 6, 8, 10, 12, 14. The elongated members are optical fibers surrounded by or coated with polyvinyl chloride (“PVC”). FIG. 9 of the '430 patent illustrates the cross-section of one of the elongated members, showing the optical fiber 1 loosely housed within a PVC tube 5 reinforced with Kevlar® strength members 3. (Kevlar® is a trademark of the Dupont Company referring to a para-aramid fiber manufactured by that company. According to the information provided by the DuPont Company through its website, http://www.dupont.com/kevlar/whatiskevlar.html, Kevlar® fibers consist of long molecular chains produced from poly-paraphenylene terephthalamide.) The elongated members 4, 6, 8, 10, 12, 14 form a mesh between two upright posts 16, 18. In one embodiment, at each joining point 20 of the elongated members 8, 10, 12, 14, the elongated members 8, 10, 12, 14 are joined together by a steel ferrule 36. The optical fibers 4, 6, 8, 10, 12, 14 are connected between an output control box 22 and an input control box 24. If the amplitude of the signals received by the output control box 22 fall outside a predetermined range, an output signal is transmitted to operate an alarm 34.
U.S. Pat. No. 4,371,869 describes a fence or wall incorporating a fiber-optic wave guide. The security system described in the '869 patent includes a composite strip 1 of bendable material, which is referred to as a carrier strip 2. The carrier strip 2 may be made of steel, for example. A single fiber-optic filament 3 extends within a groove running along the face of the carrier strip 2. The carrier strip 2 preferably is coated with a corrosion-resistant layer in the form of a sheathing 4, for example. A light source or laser generator 6 directs light into one end of the fiber-optic filament 3 and a detector 7 receives the light signal at the other end. The detector 7 is connected to a warning device 8. The warning device 8 is activated if a change in the intensity of light exiting the fiber-optic filament 3 falls outside of a predetermined range.
U.S. Pat. No. 4,450,434 describes an apparatus for determining a break in locations in fencing. As described, a cable 4 is strung along a fence 2 and is attached to the fence 2 via support means 6 (or is attached to the fence by being interwoven therein). The cable 4 is made of an optical fiber with an electrical transmission line running therealong. In the embodiment illustrated in FIG. 2, the cable 4 is made of two optical fibers 8, 10, which are coated with metallized coatings 12, 14. The coated fibers are separated from one another by the insulating material 16 in which they are embedded. The cable 4 is connected to an LED or laser transmitter 30 at one end and a light receiver 32 at the other end. One end of the transmission line is connected to a pulse generator 34 and a pulse receiver 36. The other end is open-circuited. The optical portion of the cable 4 carries the light signal such that a break in the light signal is detected by the level detector 40, triggering operation of an alarm means 42. The output of the level detector 40 is connected to the input of the electrical pulse generator 34, which generates an electrical pulse that is reflected to the pulse receiver 36. The time delay between the initiation and receipt of the electrical pulse permits the system to establish the location of the break.
U.S. Pat. No. 4,558,308 describes an intrusion warning wire-lattice that comprises a number of single fence sections 1 mounted between box-type posts 2. Each fence section 1 includes solid wires (shown in dashed lines) and a serpentine coil 4 made of a hollow wire (shown in solid lines). The tubular wires encapsulate an electrical or optical conductor 8 that is connected to a alarm system, which is responsive to a break or deformation of the electrical or optical conductor 8.
U.S. Pat. No. 4,829,286 describes a security fence system made up of a taut wire fence made of taut wires 10 strung in a parallel orientation between an anchoring post 12 and a sensor post 14. An optical fiber 17 is threaded serially through a plurality of adjacent sensor posts and is coupled to a signal transceiver 18. A taut wire connection element 34 is connected to the sensor post 14 and to two adjacent taut wires 36. The connection element 34 rotatably connects to the sensor post 14. An optical fiber engagement member 38 connects to the rod 30 so that is also rotates on the sensor post 14. The taut wire connection element 34 and the optical fiber engagement member 38 are connected to one another such that, if the taut wire connection element 34 and the optical fiber engagement member 38 rotate with respect to one another over a long period of time, the optical fiber 42 is not disturbed. As a result, no alarm sounds. However, if the taut wire connection element 34 is rotated rapidly, the optical fiber engagement member 38 also rotates, displacing the optical fiber 42, thereby triggering an alarm.
U.S. Pat. No. 5,530,430 describes a vibration responsive barbed tape security system. The security system is made up of multiple sections of fence barrier 1 that surround an area. The barrier includes a chain link fence 2 and a secondary barrier 4 in the form of a spiral barbed tape. A tube 24 is in contact with each loop of the spiral barbed tape. The tube 24 contains a vibration sensitive fiber optic cable 30 that transmits light in a predictable manner. Any movement or vibration of the tube vibrates the fiber optic cable, triggering an alarm.
UK Patent Application No. GB 2 038 060 describes an intruder alarm that gives an alarm when an intrusion has occurred into a protected area and also gives an alarm when an attempt to breach a protected area is in progress. The intruder alarm includes a network of optical fiber light conductors forming or included in a fence. Light is fed into the ones of the light conductors such that penetration or attempted penetration of the light conductors triggers an alarm.
According to the English translation, Japanese Patent No. JP 3053400 describes a trespasser monitor method that detects whether a trespasser exists and the position of the trespasser by detecting the position of the reflection of an optical pulse from a disconnection point.
Prior art security systems that rely at least in part on optical fiber technology have not been widely accepted because of certain disadvantages inherent in those systems. In particular, many prior art systems are prone to produce an unacceptably high error rate in detecting incursions into a protected perimeter. Specifically, prior art systems typically signal alarms even though a breach of the secure perimeter has not occurred. In addition, prior art systems typically are very expensive. This is due, at least in part, to the number or type of optical generator or receivers that are needed for the systems to operate.
These difficulties with the prior art, among others, cry out for a solution that the present invention provides.